1. Field of the Invention
The present invention relates to a flow rate measuring apparatus, and relates to an apparatus for measuring mass flow rate of air in an internal combustion engine, for example.
2. Description of the Related Art
Conventional flow rate measuring apparatuses include a bypass passage that bends a plurality of times from an inflow aperture to an outflow aperture, and a flow rate detecting element on which is formed a thermosensitive resistor constituted by a material such as platinum, etc., is disposed internally, and foreign matter such as dust, etc., contained in the measured fluid is decelerated and prevented from striking the flow rate detecting element at high speed by making the bypass passage bend a plurality of times. The outflow aperture of the bypass passage is disposed at a position that is further away from a circuit housing portion than the inflow aperture. (See Patent Literature 1, for example.)
Patent Literature 1: Japanese Patent No. 3602762 (Gazette)
Generally, in automotive internal-combustion engines, since cycles of intake, compression, explosion, and exhaust are performed repeatedly and such engines are constituted by a plurality of cylinders, flow velocity of intake air is a pulsating flow varying over time. Thus, main flow flowing inside a main passage of the internal-combustion engine pulsates at frequencies synchronized with rotational frequencies of the internal-combustion engine, and the lower the frequency, the more convex the radial flow velocity distribution centered on the main passage central axis, and the higher the frequency, the flatter the flow velocity distribution.
In conventional flow rate measuring apparatuses, the inflow aperture and the outflow aperture of the bypass passage are positioned so as to be separated in a radial direction of the main passage. Thus, when conventional flow rate measuring apparatuses are used in automotive internal-combustion engines, since the radial flow velocity distribution in the main passage varies depending on the rotational frequency of the internal-combustion engine even if the flow rate of the intake air flowing through the main passage is equal, differences arise between flow velocities in the vicinities of the inflow aperture and the outflow aperture of the bypass passage due to changes in the rotational frequency despite the amount of intake air being equal, giving rise to differences in the flow velocity of the flow that is induced inside the bypass passage, thereby generating errors in detected flow rate in the flow rate measuring apparatus.
Furthermore, when conventional flow rate measuring apparatuses are used in automotive internal-combustion engines, it is necessary for at least the inflow aperture and the outflow aperture to project inside the main passage, increasing projecting length in a radial direction of the main passage, thereby increasing pressure loss occurring due to the flow rate measuring apparatus being plugged into the main passage.